An environmental reconstruction of the sediment infill of the Bogota basin (Columbia) during the last 3 million years from abiotic and biotic proxies

The lacustrine sediments of the intramontane basin of Bogotá (4°N, 2550 m altitude) were collected in a 586-m deep core Funza-2. Absolute datings show sediment infill started c. 3.2 Ma and continued almost without interruptions as a result of the balance between tectonic subsidence and sediment infill. Analysis of downcore changes in lithology, grain size, facies, loss on ignition (LOI), and hydrosere vegetation at 20-cm intervals along the core produced a 2200-sample record of basin dynamics and lake level changes with a temporal resolution from c. 800 years (during the last 1.5 Ma) to c. 2000 years (from 1.5 to 3.2 Ma). We recognized 11 discrete sedimentary facies that reflect 4 different depositional environments. Facies 1 and 2 correspond to lacustrine environment where the LOI is < 20%. A swamp environment is reflected by facies 11, where the LOI is > 20%. A fluvio-lacustrine environment is represented by the facies 3, 4, 5, 7 and 9. A fluvial environment is reflected by facies 6, 8 and 10. Basin infill started with accumulation in a fluvio-lacustrine environment between 586 and 530 m below the present-day sediment surface of the basin. In the 530- to 325-m interval (c. 3-1.5 Ma), the basin contained mainly shallow water and swamps, in combination with some fluvial activity. The 325-5-m interval (1.5-0.028 Ma) shows almost uninterrupted lacustrine paleoenvironmental conditions. The uppermost 5 m reflect a fluvial paleoenvironment deposited by the Bogotá River and its tributaries. The downcore changing ratio between shallow water aquatics (Myriophyllum, Ludwigia, Polygonum, Cyperaceae) and aquatics of deeper water (Isoëtes) is indicative of lake level changes. Isoëtes as an indicator of high water levels is not consistent with fluvial and fluvio-lacustrine depositional regimes. Several lithological discontinuities, evidenced by the sharp transition from swamp deposits to fluvio-lacustrine and fluvial deposits, are present in the core, in the interval from 530 to 325 m in particular. It is plausible that swamp deposits have been eroded during episodes during which the water table quickly raised or during events of sudden coarse fluvial input into the basin. © 2005 Elsevier B.V. All rights reserved.